The hair cells of the inner ear transduce sound into the neuronal signals that give rise to hearing. The differentiation of hair cells is therefore necessary for normal hearing, and conversely its disruption can lead to deafness. This project will investigate factors that control the differentiation of hair cells from supporting cells, during the development of the mammalian inner ear. Intercellular communication by Delta-Notch signaling has been proposed as a major regulator of the differentiation of these two cell types. In order to explore the role of this signaling pathway, transgenic models will be generated which exhibit disabled Delta-Notch signaling during the period of developmental differentiation of hair cells and supporting cells. A promoter construct derived from for the POU-domain gene Brn-3.1 will be used to drive the expression of a constituitively active intracellular Notch receptor fragment, or a truncated dominant negative Delta-1 ligand. Expression under the control of this promoter will be limited to hair cells and their precursors, beginning before there is any detectable morphological differentiation of the two cell types. The development of both morphological and molecular features of hair cells and supporting cells will be evaluated in these transgenic models. The results will provide evidence regarding the importance of Delta-Notch signaling in the development of these cellular phenotypes in the sensory epithelia of the inner ear.